Luminescent materials, termed phosphors, have general utility in luminescent displays. A phenomenon common to all phosphors is their ability to emit photons that are visible to the human eye when the phosphors are excited to elevated energy levels. One excitation technique employed in many luminescent displays, including cathode ray tubes, vacuum fluorescent displays, and field emission displays, projects electrons through a vacuum onto a display screen from an electron-emitting cathode positioned proximal to the display screen. When the cathode is activated, electrons traveling from the cathode strike phosphors positioned on the display screen. The electrons are reflected, scattered or absorbed on contact with the display screen. The incident electrons transfer energy to the phosphors, thereby exciting the phosphors and advantageously causing them to emit visible light. By selectively exciting the phosphors in a given pattern, an image can be created on the display screen.
Other types of luminescent displays rely on substantially the same principle as described above to create images on the display screen, but excite the phosphors by alternate forms of energy, such as x-rays, gamma rays, and ultra-violet, to name a few. In any case, many luminescent displays, including cathode ray tubes, field emission displays and plasma displays, require high resolution display screens for optimum performance. It has been found that screen resolution is inter alia a function of the average phosphor particle size and the phosphor particle size distribution. In particular, it has been found that extremely fine grain phosphors having a relatively small average particle size and a narrow particle size distribution are a prerequisite for satisfactory high resolution display screens.
The conventional method for manufacturing phosphors is to mix the phosphor starting materials in dry particulate form and fire the resulting mixture in an oven or a kiln at a high temperature, transforming the starting materials to the desired phosphor composition. This method, however, produces large grain phosphors having a relatively large average particle size and a wide particle size distribution, characteristics that are unsatisfactory for high resolution display screen applications. Accordingly, the large grain phosphors produced in this manner are mechanically sized by milling, crushing and sieving, or other such means to obtain the desired fine grain phosphors. Unfortunately, mechanical sizing of the phosphor particles diminishes the luminescent efficiency of the resulting fine grain phosphors, correspondingly diminishing the luminance of display screens employing such phosphors. In addition, the resolution of such display screens is also often diminished since a high current density may be required to achieve the requisite luminance of the display screens.
As such, a need exists for extremely fine grain phosphors having satisfactory performance characteristics for use in high resolution display screen applications. Accordingly, it is an object of the present invention to provide a process for manufacturing a phosphor that satisfies the performance demands of high resolution display screens. It is another object of the present invention to provide a process for manufacturing a phosphor that enables precise control of the particle size and luminescent characteristics of the resulting phosphor. More particularly, it is an object of the present invention to provide a process for manufacturing an extremely fine grain phosphor that does not require mechanical sizing of the phosphor product which would diminish the luminescent efficiency thereof. These objects and others are accomplished in accordance with the invention described hereafter.